It has been demonstrated that metallic contamination, such as flakes of plating, light metallic turnings, or small coil-shaped pieces from tapped holes in electrical equipment are responsive to the electrostatic field in the gas gap between the high-potential electrode parts, and the outer grounded metallic tank, particularly used in gas-filled circuit breakers. At voltage stress levels below normal service-stress levels, this contamination assumes a charge, and commonly aligns with the electrical voltage gradient. Some shapes or pieces move at random about the floor of the surrounding tank, and others may levitate to contact the upper end of other moving particles, or attach themselves to the high-potential electrode, or contact parts, where again they may wander at random with a luminous discharge at the point of contact. Breakdown sometimes occurs at voltage stresses below the normal line-to-ground voltage of the circuit-breaker application. Such phenomena occur in gas-insulated circuit breakers and gas-insulated transmission lines.
In recent years, there has been an effort to reduce the installation land area for substations due to the expense of real estate, or land near large urban areas, and a line of gas-insulated transmission equipment has been designed and sold to supply this utility need for reduced installation areas. The following patents illustrate the general types of equipment involved, and set forth descriptions of pieces, or components of gas-insulated equipment suitable for supplying relatively heavy currents at high voltages within relatively small enclosures: U.S. Pat. No. 3,448,202, D. L. Whitehead; U.S. Pat. No. 3,361,870, D. L. Whitehead; U.S. Pat. No. 3,610,807, D. L. Whitehead; U.S. Pat. No. 3,378,731, D. L. Whitehead; U.S. Pat. No. 3,331,911, D. L. Whitehead; U.S. Pat. No. 2,173,717, H. M. Hobart; U.S. Pat. No. 2,221,670, F. S. Cooper; U.S. Pat. No. 3,515,939, J. G. Trump; U.S. Pat. No. 2,216,010, H. M. Hobart; U.S. Pat. No. 3,345,450, H. E. Spindle; U.S. Pat. No. 2,221,671, F. S. Cooper; and U.S. Pat. No. 3,585,270, J. G. Trump.
In equipment described in the patents above, the problem exists with small metallic or insulating particles being present and reducing the breakdown voltage between metallic parts at different voltage levels, or reducing parts at different voltage levels, or reducing the flashover voltage along insulating surfaces, particularly along support insulators or spacers.
One method utilized in the prior art to overcome this problem is present in U.S. Pat. No. 3,856,978, and its divisional, U.S. Pat. No. 3,911,937, To Sletten et al. These patents describe the use of either thermosetting or polyvinyl copolymer coatings which are placed on selected surface areas within gas-insulated apparatus. The coating of these patents is initially hard, but is rendered sticky or tacky either by heating means or by the introduction of a solvent vapor. However, one possible disadvantage of the coatings described in these patents is that, for use with a solvent vapor, the ability of this coating to remain tacky for extended periods of time is unknown. In the gas-insulated apparatus in which the invention is utilized, it may be desirable to provide the tacky coating to entrap particles for period of time up to, for example, twenty years. One problem with the coatings described in the two patents is that, as the solvent vapor causes the coating to become tacky, over longer periods of time the coating may possibly be attacked throughout and thereby sag. If this occurs, the coating would, instead of maintaining their locations where initially placed, may instead, by forces of gravity, sag or slide to the lowermost portions of the gas-insulated apparatus. This sagging results from the fact that the solvent vapor, because of its interaction with the coating to render its surface tacky, may interact with the entire coating, and not just its surface, and cause the entire coating to flow.